In modern wireless communication standards, a couple of frequency bands are defined by their respective middle frequencies, bandwidths and duplex distance. A respective frequency band may comprise a Tx band that is used for transmission of signals at a user's phone, the direction is also called upstream, and an Rx band used for reception of signals, the direction is also called downstream. The band distance describes a frequency interval separating the frequencies of a transmission signal and a receive signal. A band pass filter used for a Tx or a Rx band needs a steep skirt a the side of the pass band that is facing the adjacent Rx or Tx band such that the attenuation in the adjoining band is above the required value.
The growing demand for more bandwidth necessary for new wireless services results in challenging frequency band definitions. One example is band 28 being an APEC specific application. The Tx band is from 703 to 748 MHz and the respective Rx band from 758 to 803 MHz complying with a bandwidth of 6% and a band distance of 10 MHz. This definition is challenging because the two requirements of a large bandwidth of nearly 6% and a small band distance of 10 MHz are nearly impossible to be realized at the same time by a front-end circuit based on filters working with surface acoustic waves. A further challenge is the temperature coefficient of frequency (TCF) which means that filter properties such as middle frequencies and hence, filter skirts too change with temperature. Hence, in spite of a given TCF a filter must obey the definitions of a band at each temperature within a temperature interval complying with normal environmental conditions.
An electro-acoustic band pass filter realized on a standard lithium tantalate piezoelectric material complies with the required small band distance and the skirt steepness resulting therefrom. But such a filter cannot satisfy the bandwidth requirement and is far from reaching it. On the other side, a band pass filter realized on a standard lithium niobate piezoelectric material can reach a bandwidth of about 6% but is far from complying with the demands in view of the skirt steepness. This is due to its relatively large TCF (temperature coefficient of frequency) of about 80 ppm/K.